Phase shifting network

ABSTRACT

A phase shifting network for an inductor motor is provided. The network includes a resistor connected in series with each motor phase coil of a pair of motor phases. The network further includes a capacitor connected between the motor phases. The inventive phase shifting network produces current and voltage waveforms within the motor phase coils that are smoother, more equal, and less subject to harmonic distortion as compared to the waveforms generated by conventional phase shifting networks.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a control circuit for an inductor motorand, in particular, to a control circuit that functions as a phaseshifting network and enables a motor to run more smoothly by producingcurrent and voltage waveforms within the respective motor phases thatare smoother, more equal and less subject to harmonic distortion.

[0003] 2. Disclosure of Related Art

[0004] A conventional control circuit 10 for use as a phase shiftingnetwork in a two-phase inductor motor is shown in FIG. 2A. Circuit 10includes a resistor 12 and a capacitor 14 connected in series between apair of motor phases 16, 18. Each of motor phases 16, 18 includes amotor phase coil 20, 22, respectively. A directional switching device 24is used to control the sequence of energization of phase coils 20, 22.FIGS. 3A-6A illustrate the various current and voltage waveforms presentin motor phases 16, 18 during operation of a motor incorporating circuit10. In particular, FIGS. 3A and 5A illustrate current and voltagewaveforms, respectively, present in motor phases 16, 18 during normaloperation of the motor. FIGS. 4A and 6A illustrate current and voltagewaveforms, respectively, present in motor phases 16, 18 as a breakdownin motor torque is about to occur. The current and voltage waveforms forphase 16 of circuit 10 are shown in a solid line while the current andvoltage waveforms for phase 18 of circuit 10 are shown in broken line.It should be noted that FIGS. 3A-6A illustrate energization of phases16, 18 in the sequence 16→18 (i.e., with the current and voltagewaveforms of phase 18 phase-shifted relative to phase 16). Asillustrated in FIGS. 3A-6A, the current and voltage waveforms withineach individual phase 16, 18 of circuit 10 are subject to relativelylarge variations in magnitude. Moreover, the magnitude of the currentand voltage within phase 16 varies significantly from the magnitude ofthe current and voltage, respectively, within phase 18. Finally, thevoltage in phases 16, 18 is at times subject to a relatively largeamount of harmonic distortion as shown in FIG. 6A. These deficienciesresult in torque pulses within a motor incorporating circuit 10, therebycausing the velocity of the motor to modulate and the motor to runrough.

[0005] There is thus a need for a control circuit for a motor that willminimize or eliminate one or more of the above-mentioned deficiencies.

SUMMARY OF THE INVENTION

[0006] The present invention provides a control circuit for use as aphase shifting network in a motor such as an inductor motor.

[0007] An object of the present invention is to provide a controlcircuit for a motor that will reduce velocity modulation in the motorand thereby enable smoother operation of the motor.

[0008] Related objects of the present invention are to provide a controlcircuit for a motor that will produce current and voltage waveformswithin the motor phases that are smoother, more equal, and less subjectto harmonic distortion as compared to the current and voltage waveformsgenerated by conventional control circuits.

[0009] A control circuit for a motor in accordance with the presentinvention includes a first motor phase having a first resistor connectedin series with a first phase coil of the motor. The circuit furtherincludes a second motor phase having a second resistor connected inseries with a second phase coil of the motor. Finally, the circuitincludes a capacitor connected between the first and second motorphases.

[0010] A control circuit in accordance with the present inventionsmooths the current and voltage waveforms within the motor phase coilsby reducing the non-linear characteristics of the motor. First, thecontrol circuit reduces the maximum operating voltage of the motorthereby preventing the magnetic structure of the motor from saturating.Second, the addition of linear impedance devices such as resistors inseries with each phase coil makes the motor more linear than the motoralone. A control circuit in accordance with the present invention alsoequalizes the magnitude of the current in the motor phases coils—therebyproducing a smoother running motor—by equalizing the circuit impedancebetween the phases of the motor. In a conventional phase shiftingnetwork such as circuit 10 in FIG. 2A, the impedance between phases 16,18 differs by the impedance of a resistor 12 and a capacitor 14. In theinventive control circuit, the difference in impedance between thephases is limited to the impedance of the capacitor (because both phasesinclude a resistor).

[0011] The inventive control circuit has several additional advantagesas compared to conventional control circuits. First, the inventivecircuit results in a lower operating voltage for the motor, therebyallowing the use of wire of various diameters within the motor phases.The use of larger diameter wire can be advantageous because largerdiameter wire is easier to wind and to terminate. Second, the inventivecircuit enables the motor to run cooler for a given supply voltage ascompared to conventional circuits. Finally, the inventive circuitenables a motor to start loads having a larger inertia as compared toconventional circuits.

[0012] These and other features and objects of this invention willbecome apparent to one skilled in the art from the following detaileddescription and the accompanying drawings illustrating features of thisinvention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a perspective view of an inductor motor.

[0014]FIG. 2A is a schematic diagram of a conventional control circuitfor use as a phase shifting network.

[0015]FIG. 2B is a schematic diagram of a control circuit for use as aphase shifting network in accordance with the present invention.

[0016]FIG. 3A is a waveform diagram illustrating the current waveformsin the respective motor phases of a motor incorporating the circuit ofFIG. 2A during normal operation.

[0017]FIG. 3B is a waveform diagram illustrating the current waveformsin the respective motor phases of a motor incorporating the circuit ofFIG. 2B during normal operation.

[0018]FIG. 4A is a waveform diagram illustrating the current waveformsin the respective motor phases of a motor incorporating the circuit ofFIG. 2A during torque breakdown.

[0019]FIG. 4B is a waveform diagram illustrating the current waveformsin the respective motor phases of a motor incorporating the circuit ofFIG. 2B during torque breakdown

[0020]FIG. 5A is a waveform diagram illustrating the voltage waveformsin the respective motor phases of a motor incorporating the circuit ofFIG. 2A during normal operation.

[0021]FIG. 5B is a waveform diagram illustrating the voltage waveformsin the respective motor phases of a motor incorporating the circuit ofFIG. 2B during normal operation.

[0022]FIG. 6A is a waveform diagram illustrating the voltage waveformsin the respective motor phases of a motor incorporating the circuit ofFIG. 2A during torque breakdown.

[0023]FIG. 6B is a waveform diagram illustrating the voltage waveformsin the respective motor phases of a motor incorporating the circuit ofFIG. 2B during torque breakdown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] Referring now to the drawings wherein like reference numerals areused to identify identical components in the various views, FIG. 1illustrates a typical inductor motor 26. Although the illustrated motorcomprises a two-phase inductor motor, it should be understood that theinvention as disclosed herein could be applied to other motors as isknown in the art. Motor 26 includes a rotor assembly 28 and a statorassembly 30, both of which may be centered about an axis 32.

[0025] Rotor assembly 28 is provided to move a load (not shown)connected to rotor assembly 28. Assembly 28 includes a shaft 34 and arotor 36 disposed about shaft 34. Shaft 34 is provided to engage eitherthe load or another means for engaging the load. Shaft 34 extendslongitudinally along axis 32 and may be centered about axis 32. Rotor 36is provided to impart rotation to shaft 34 and is capable of clockwiseor counterclockwise rotation. Rotor 36 may be comprised of a pluralityof laminations of a material having relatively low magnetic reluctance,such as iron. Rotor 36 may be centered about axis 32 and may include aspline or key (not shown) configured to be inserted within a keyway (notshown) in shaft 34. Rotor 36 includes a plurality of radially outwardlyextending rotor poles 38 configured as a diametrically opposed rotorpole pair a-a′. Each of poles 38 is generally rectangular incross-section and may include one or more radially outwardly extendingteeth as is known in the art. It will be understood by those skilled inthe art that the number of poles 38 of rotor 36 may vary.

[0026] Stator assembly 30 is provided to produce a torque to causerotation of rotor assembly 28. Stator assembly 30 may comprise aplurality of laminations 40 that are formed from a material, such asiron, having a relatively low magnetic reluctance. Assembly 30 includesa plurality of radially inwardly extending poles 42 configured asdiametrically opposed stator pole pairs A-A′, B-B′. Each pair of statorpoles 42 is provided to attract a corresponding pair of rotor poles 38of rotor assembly 28 and thereby cause rotation of rotor assembly 28.Poles 42 are generally rectangular in cross-section and may include oneor more radially inwardly extending teeth (not shown) as is known in theart. Poles 42 may extend along the axial length of stator assembly 30and define a bore 44 that is adapted to receive rotor assembly 28 . Itwill be understood by those in the art that the number of stator poles42 may vary.

[0027] Rotation of rotor assembly 28 is produced by energizing phasecoils 46, 48 surrounding each stator pole pair. Phase coils 46, 48 areformed by connecting, in series or in parallel, windings ondiametrically opposed stator poles 42. As one of phase coils 46, 48begins to conduct current, the nearest rotor pole pair is magneticallyattracted toward the stator pole pair around which the energized phasecoil is wound.

[0028] Referring now to FIG. 2B, a control circuit 50 in accordance withthe present invention is provided. Circuit 50 includes a pair of motorphases 52, 54. Each motor phase 52, 54 includes a phase coil 46, 48.Circuit 50 further includes means, such as capacitor 56 and resistors58, 60, respectively, for controlling a first phase current in phasecoil 46 and a second phase current in phase coil 48. Circuit 50 mayfurther include means, such as directional switching device 62, forselectively energizing phase coils 46, 48 in a plurality of phasesequences to thereby change the direction of motor 26.

[0029] Capacitor 56 is provided to generate a phase shift in the voltageand current supplied to one of phase coils 46, 48 by a single-phasepower source 64. Capacitor 56 is conventional in the art. Capacitor 56is connected between motor phases 52, 54, having a first plate 66connected to a node 68 and a second plate 70 connected to a node 72.

[0030] Resistors 58, 60 are provided to reduce velocity modulationwithin motor 26 by smoothing the current and voltage waveforms withinmotor phases 52, 54, equalizing the velocity and current magnitudes inthe respective motor phases 52, 54, and reducing harmonic distortion ofthe voltage waveforms within motor phases 52, 54. Resistor 58 isconnected in series with phase coil 46, having a first end connected toplate 66 of capacitor 56 at node 70 and a second end connected to phasecoil 46. Resistor 60 is connected in series with phase coil 48, having afirst end connected to plate 70 of capacitor 56 at node 72 and a secondend connected to phase coil 48.

[0031] Directional switching device 62 is provided to enableenergization of phase coils 46, 48 in multiple sequences so that rotorassembly 28 can be made to rotate in either a clockwise orcounterclockwise direction. Switching device 62 is conventional in theart.

[0032] Referring now to FIGS. 3A-6A and 3B-6B, the operative effect of acontrol circuit 50 in accordance with the present invention will bedescribed. FIGS. 3A-6A illustrate current and voltage waveforms in amotor incorporating conventional control circuit 10. The current andvoltage waveforms in motor phase 16 are shown in solid line while thecurrent and voltage waveforms in motor phase 18 are shown in brokenline. FIGS. 3B-6B illustrate current and voltage waveforms in a motor 26incorporating inventive circuit 50. The current and voltage waveforms inmotor phase 52 are shown in solid line while the current and voltagewaveforms in motor phase 54 are shown in broken line. Although thewaveforms assume a sequence of energization of 16→18 in circuit 10 and52→54 in circuit 50, it should be understood that the sequence ofenergization may vary.

[0033] As shown in FIGS. 3A, during normal operation of a motorincorporating conventional control circuit 10, the currents in phasecoils 20, 22 reach magnitudes of I_(A1) and I_(A2) amperes,respectively. The difference in peak current magnitudes between phases16, 18 is therefore Δ(|I_(A1)-I_(A2)|). Referring now to FIG. 3B, duringnormal operation of a motor 26 incorporating a control circuit 50 inaccordance with the present invention, the currents in phase coils 46,48 reach magnitudes of I_(B1) and I_(B2) amperes, respectively. I_(B1)and I_(B2) are less than I_(A1) and I_(A2), respectively. As a result,the current waveforms in phases 52, 54 of circuit 50 are smoother thanthe current waveforms in phases 16, 18 of conventional circuit 10. Moreimportantly, the difference in peak current magnitudes between phases52, 54, Δ(|I_(B1)-I_(B2)|), is less than Δ(I_(A1)-I_(A2)|). Because thepeak current magnitudes in phases 52, 54 are more equal, there is lessvelocity modulation within motor 26.

[0034] The same results are achieved as the motor approaches a breakdownin motor torque. Referring to FIG. 4A, in a motor incorporatingconventional control circuit 10, the currents in phase coils 20, 22reach magnitudes of I_(A3) and I_(A4) amperes, respectively. Thedifference in peak current magnitudes between phases 16, 18 is thereforeΔ(I_(A3)-I_(A4)|). Referring now to FIG. 4B, in a motor 26 incorporatinga control circuit 50 in accordance with the present invention, thecurrents in phase coils 46, 48 reach magnitudes of IB3 and I_(B4)amperes, respectively. I_(B3) and I_(B4) are less than I_(A3) andI_(A4), respectively. As a result, the current waveforms in phases 52,54 of circuit 50 are smoother than the current waveforms in phases 16,18 of conventional circuit 10. More importantly, the difference in peakcurrent magnitudes between phases 52, 54, Δ(|I_(B3)-I_(B4)|), is lessthan Δ(|I_(A3)-I_(A4)|). Because the peak current magnitudes in phases52, 54 are more equal, there is again less velocity modulation withinmotor 26.

[0035] Referring now to FIGS. 5A and 5B, during normal operation ofmotors incorporating conventional control circuit 10 (FIG. 5A) andinventive circuit 50 (FIG. 5B) the voltage waveforms within phases 16,18 of circuit 10 and phases 52, 54 of circuit 50 are substantiallyequivalent. However, as the motors approaches torque breakdown, thevoltage waveforms differ as shown in FIGS. 6A and 6B. Referring to FIG.6A, in a motor incorporating conventional control circuit 10, thevoltages in phases 16, 18 reach magnitudes of V_(A3) and V_(A4) volts,respectively. The difference in peak voltage magnitudes between phases16, 18 is therefore Δ(|V_(A3)-V_(A4)|). Referring now to FIG. 6B, in amotor 26 incorporating a control circuit 50 in accordance with thepresent invention, the voltages in phases 52, 54 reach magnitudes ofV_(B3) and V_(B4) volts, respectively. V_(B3) and V_(B4) are less thanV_(A3) and V_(A4), respectively. As a result, the voltage waveforms inphases 52, 54 of circuit 50 are smoother than the voltage waveforms inphases 16, 18 of conventional circuit 10. More importantly, thedifference in peak voltage magnitudes between phases 52, 54,Δ(|V_(B3)-V_(B4)|), is less than Δ(|V_(A3)-V_(A4)|). Because the peakvoltage magnitudes in phases 52, 54 are more equal, there is again lessvelocity modulation within motor 26. Finally, the voltage waveforms inphases 52, 54 of circuit 50 do not suffer from the same level ofharmonic distortion as the voltage waveforms in phases 16, 18 of circuit10 as illustrated in FIGS. 6B and 6B.

[0036] A control circuit 50 in accordance with the present inventionprovides smoother and more equal current and voltage waveforms withinthe motor phases 52, 54 to thereby reduce velocity modulation withinmotor 26. Circuit 50 smoothes the current and voltage waveforms byreducing the non-linear characteristics of motor 26. First, controlcircuit 50 reduces the maximum operating voltage of motor 26 therebypreventing the magnetic structure of motor 26 from saturating. Second,the addition of resistors 58, 60 increases the linear impedance of motor26. Circuit 50 equalizes the magnitude of the current in the motorphases coils 46, 48—thereby producing a smoother running motor 26—byequalizing the circuit impedance between motor phases 52, 54. Asmentioned hereinabove, in a conventional phase shifting network such ascircuit 10 in FIG. 2A, the impedance between the phases 16, 18 differsby the impedance of a resistor 12 and a capacitor 14. In circuit 50, thedifference in impedance between phases 52, 54 is limited to theimpedance of capacitor 56 (because both phases 52, 54 include resistors58, 60).

[0037] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it is wellunderstood by those skilled in the art that various changes andmodifications can be made in the invention without departing from thespirit and scope of the invention.

We claim:
 1. A control circuit for a motor, comprising: a first motor phase having a first phase coil and a first resistor connected in series; a second motor phase having a second phase coil and a second resistor connected in series; and, a capacitor connected between said first and second motor phases.
 2. The control circuit of claim 1 wherein said first and second phase coils are selectively energized by a single-phase power source and a first phase current in said first phase coil is phase-shifted relative to a second phase current in said second phase coil.
 3. The control circuit of claim 2 wherein the magnitude of said first phase current is substantially equal to the magnitude of said second phase current.
 4. The control circuit of claim 1, further comprising means for selectively energizing said first and second phase coils in a plurality of phase sequences.
 5. A control circuit for a motor, comprising: a first motor phase having a first phase coil; a second motor phase having a second phase coil; and, means for controlling a first phase current in said first phase coil and a second phase current in said second phase coil wherein said first and second phase coils are selectively energized by a single-phase power source and said first phase current is phase-shifted relative to said second phase current.
 6. The control circuit of claim 5 wherein said controlling means includes: a first resistor connected in series with said first phase coil; a second resistor connected in series with said second phase coil; and, a capacitor connected between said first and second motor phases.
 7. The control circuit of claim 5 wherein the magnitude of said first phase current is substantially equal to the magnitude of said second phase current.
 8. The control circuit of claim 5, further comprising means for selectively energizing said first and second phase coils in a plurality of phase sequences.
 9. A motor, comprising: a stator having first and second pairs of diametrically opposed stator poles; and, a control circuit including a first motor phase having a first resistor connected in series with a first phase coil, said first phase coil disposed about said first stator pole pair; a second motor phase having a second resistor connected in series with a second phase coil, said second phase coil disposed about said second stator pole pair; and, a capacitor connected between said first and second motor phases.
 10. The control circuit of claim 9 wherein said first and second phase coils are selectively energized by a single-phase power source and a first phase current in said first phase coil is phase-shifted relative to a second phase current in said second phase coil.
 11. The control circuit of claim 10 wherein the magnitude of said first phase current is substantially equal to the magnitude of said second phase current.
 12. The control circuit of claim 9, further comprising means for selectively energizing said first and second phase coils in a plurality of phase sequences. 